Eye lens opacification, or senile cataract appears to be the result of, and is associated with, the aggregation and precipitation of damaged lens proteins (1). In contrast to the observed accumulation of damaged proteins in the aged lens, many cells contain components which can catabolize damaged (as opposed to native) proteins to their consistuent amino acids much more rapidly (2-6). The damaged proteins are identified for degradation by covalent attachment of several ubiquitin molecules. After such recognition and initiation these ubiquitin conjugates are very rapidly removed by a cytoplasmic, ATP stimulated, proteolytic pathway (2-6). It is plausable that an absence of sufficient lenticular ubiquitin, or a paucity of enzymes associated with the ubiquitin dependent pathway, results in the observed inability of the aged lens to effectively catabolize, or initiate the editing of, damaged lenticular proteins. These circumstances might also be associated with the slow turnover of proteins which is particularly obvious in the core of the normal lens (1,7). Substantial work on the lenticular endopeptidases has been reported, but there are no reports on the mechanism of recognition of proteins as substrates for the lenticular proteases and there are no reports about energy requiring, ubiquitin stimulated lens proteases (8-12). Preliminary tests show that the lens contains a component which reacts with antibodies prepared against ubiquitin. It is the objective of this research to determine by immunofluorescence the localization of ubiquitin in young and old hog lens. This should help identify those lens zones in which ubiquitin dependent selective proteolysis of damaged proteins can occur in normal lens tissue and will provide circumstantial evidence regarding the presence of (or absence of) ubiquitin requiring, ATP stimulated, proteolytic processes which seem to be necessary for the rapid removal of damaged, precatractous proteins. Thus it is hoped that the results of this research will stimulate a search for those proteases or cofactors which are involved in the recognition of damaged lens proteins and in the initiation of the lenticular proteolytic process.